You might be able to make something that looks like marmalade (only redder) but it won't taste like marmalade as it will not have the bitterness/sourness that the Seville orange brings. I made some Mandarin "Marmalade" a few years ago. It was quite nice, but not real marmalade. It would be interesting to hear how it turns out.
It will be fine, maybe add some extra lemon and reduce the sugar a little. I've made marmalade with all sorts before - lemon, pink grapefruit etc.
For normal Seville orange, I use 3 bags of preserving sugar to one 1kg bag of oranges, plus the juice of a few lemons. I also add about a litre or so of water, as I don't like it too peely.
Here's a pic of mine from this year.
I boil the fruit whole for 2 hours, then cut in half depip and then shove in food processor and pulse so it is shredded. You can cut by hand first then boil but that takes ages. I am lazy. Then add lemon juice, peel from processor and all the sugar and water to a preserving pan and stir gently until sugar dissolved. Put some saucers in the freezer before you start this. Then boil it up rapidly for about 15 minutes, test for set by plonking a blob on frozen saucer, leave for 30 secs and see if it wrinkles when you push it. That's how to tell it's done. Sometimes it needs up to 30 minutes.
Cheers for that - I'll post a picture tomorrow when I make it!
Your marmalade looks lovely Taffeta next year I'll going to try and remember when Seville season is and make some "normal" as well but completely forgot this year! I was looking at how much jam we had left (only 4 jars!) and thought I'd better make something.